The present invention relates to a liquid crystal device for use in a liquid crystal display device, an optical shutter array, etc., and more particularly to a liquid crystal device having improved display and driving characteristics, because of improved initial alignment or orientation of liquid crystal molecules.
Hitherto, liquid crystal display devices are well known, which comprise a group of scanning electrodes and a group of signal electrodes arranged in a matrix manner, and a liquid crystal compound is filled between the electrode groups to form a plurality of picture elements thereby to display images or information. These display devices employ a time-sharing driving method which comprises the steps of selectively applying address signals sequentially and cyclically to the group of scanning electrodes, and parallely effecting selective application of predetermined information signals to the group of signal electrodes in synchronism with address signals. However, these display devices and the driving method therefore have a serious drawback as will be described below.
Namely, the drawback is that it is difficult to obtain a high density of picture elements or a large image area. Because of relatively high response speed and low power dissipation, among prior art liquid crystals, most of liquid crystals which have been put into practice as display devices are TN (twisted nematic) type liquid crystals, as shown in "Voltage-Dependent Optical Activity of a Twisted Nematic Liquid Crystal" by M. Schadt and W. Helfrich, Applied Physics Letters Vol. 18, No. 4 (Feb. 15, 1971) pp. 127-128. In the liquid crystals of this type, molecules of nematic liquid crystal which show positive dielectric anisotropy under no application of an electric field form a structure twisted in the thickness direction of liquid crystal layers (helical structure), and molecules of these liquid crystals are aligned or oriented parallel to each other in the surfaces of both electrodes. On the other hand, nematic liquid crystals which show positive dielectric anisotropy under application of an electric field are oriented or aligned in the direction of the electric field. Thus, they can cause optical modulation. When display devices of a matrix electrode array are designed using liquid crystals of this type, a voltage higher than a threshold level required for aligning liquid crystal molecules in the direction perpendicular to electrode surfaces is applied to areas (selected points) where scanning electrodes and signal electrodes are selected at a time, whereas a voltage is not applied to areas (non-selected points) where scanning electrodes and signal electrodes are not selected and, accordingly, the liquid crystal molecules are stably aligned parallel to the electrode surfaces. When linear polarizers arranged in a cross-nicol relationship, i.e., with their polarizing axes being substantially perpendicular to each other, are arranged on the upper and lower sides of a liquid crystal cell thus formed, a light does not transmit at selected points while it transmits at non-selected points. Thus, the liquid crystal cell can function as an image device.
However, when a matrix electrode structure is constituted, a certain electric field is applied to regions where scanning electrodes are selected and signal electrodes are not selected or regions where scanning electrodes are not selected and signal electrodes are selected (which regions are so called "half-selected points"). If the difference between a voltage applied to the selected points and a voltage applied to the half-selected points is sufficiently large, and a voltage threshold level required for allowing liquid crystal molecules to be aligned or oriented perpendicular to an electric field is set to a value therebetween, the display device normally operates. However, in fact, according as the number (N) of scanning lines increases, a time (duty ratio) during which an effective electric field is applied to one selected point when a whole image area (corresponding to one frame) is scanned decreases with a ratio of 1/N. For this reason, the larger the number of scanning lines are, the smaller is the voltage difference as an effective value applied to a selected point and non-selected points when scanning is repeatedly effected. As a result, this leads to unavoidable drawbacks of lowering of image contrast or occurrence of crosstalk. These phenomena result in problems that cannot be essentially avoided, which appear when a liquid crystal not having bistability (which shows a stable state where liquid crystal molecules are oriented or aligned in a horizontal direction with respect to electrode surfaces, but are oriented in a vertical direction only when an electric field is effectively applied) is driven, i.e., repeatedly scanned, by making use of time storage effect. To overcome these drawbacks, the voltage averaging method, the two-frequency driving method, the multiple matrix method, etc., has already been proposed. However, any method is not sufficient to overcome the above-mentioned drawbacks. As a result, it is the present state that the development of large image area or high packaging density in respect to display elements is delayed because of the fact that it is difficult to sufficiently increase the number of scanning lines.
Meanwhile, turning to the field of a printer, as means for obtaining a hard copy in response to input electric signals, a Laser Beam Printer (LBP) providing electric image signals to electrophotographic charging member in the form of lights is the most excellent in view of density of a picture element and a printing speed.
However, the LBP has drawbacks as follows:
(1) It becomes large in apparatus size.
(2) It has high speed mechanically movable parts such as a polygon scanner, resulting in noise and requirement for strict mechanical precision, etc.
In order to eliminate drawbacks stated above, a liquid crystal shutter-array is proposed as a device for changing electric signals to optical signals. When picture element signals are provided with a liquid crystal shutter-array, however, more than 4000 signal generators are required, for instance, for writing picture element signals into a length of 200 mm in a ratio of 20 dots/mm. Accordingly, in order to independently feed signals to respective signal generators, lead lines for feeding electric signals are required to be provided to all the respective signal generators, and the production has become difficult.
In view of this, another attempt is made to apply one line of image signals in a time-sharing manner with signal generators divided into a plurality of lines.
With this attempt signal feeding electrodes can be common to the plurality of signal generators, thereby enabling to remarkably decrease the number of lead wires. However, if the number (N) of lines is increased while using a liquid crystal showing no bistability as usually practised, a signal "ON" time is substantially reduced to 1/N. This results in difficulties that light quantity obtained on a photoconductive member is decreased, and a crosstalk occurs.
In order to obviate the above-mentioned drawbacks of the conventional types of liquid crystal devices, Clark and Lagerwall have proposed the use of a liquid crystal device using a bistable liquid crystal (Japanese Laid-Open patent application No. 107216/1981, U.S. Pat. No. 4,367,924, etc.). As the bistable liquid crystal, a ferroelectric liquid crystal having a chiral smectic C (SmC*) phase or H (SmH*) phase is generally used. The ferroelectric liquid crystal has bistability, i.e., has two stable states comprising a first stable state and a second stable state. Accordingly, different from the conventional TN-type liquid crystal in the above-mentioned device, the liquid crystal is oriented to the first stable state in response to one electric field vector and to the second stable state in response to the other electric field vector. Further this type of liquid crystal very quickly assumes either one of the above-mentioned two stable states in reply to an electric field applied thereto and retains the state in the absence of an electric field. By utilizing these properties, essential improvements can be attained with respect to the above-mentioned difficulties involved in the conventional TN-type liquid crystal device. This point will be explained in further detail in connection with the present invention.
However, in order that an optical modulation device using the liquid crystal having bistability could show desired operation performances, the liquid crystal interposed between a pair of parallel base plates is required to be placed in such a state of molecular arrangement that the transition between the two stable states can effectively occur, as a matter different from or a precondition of the application of an electric field. With respect to, for example, a ferroelectric liquid crystal having an SmC* or SmH* phase, there must be formed a monodomain wherein the layers of the liquid crystal are perpendicular to the face of the base plate and therefore the molecular axis of the liquid crystal is almost in parallel with the base plate face. However, in the optical modulation devices using a bistable liquid crystal, an orientation state of a liquid crystal having such a monodomain structure cannot satisfactorily be formed, whereby the optical modulation device cannot actually show sufficient performances.
For example, several methods have been proposed by Clark et al to give such an orientation state, including a method of applying a magnetic field, a method of applying a shearing force and a method of disposing a plurality of parallel ridges at small intervals. These methods have not necessarily provided satisfactory results. For example, the method of applying a magnetic field requires a large size of apparatus and is not readily compatible with a thin layer cell which is generally excellent in operation performances. The method of applying a shearing force is not compatible with a method where a cell structure is first formed and then a liquid crystal is poured thereinto. On the other hand, the method of disposing parallel ridges in a cell cannot impart a stable orientation effect by itself.